EDIT:
Okay this is wierd. I was just typing a response to you're post and it worked... but it didn't before...
I added the -out"directory" command as you said, and i got no result. I then removed the -out"directory" command, and it worked?!
I have umodel.exe in my documents, along with a shortcut to the umodel.exe. This is the entire command line from the shortcut:
C:\Users\James\Documents\umodel.exe -path="D:\Program Files (x86)\Unreal Tournament 3" -export -all LT_Trims
The files are being exported to My Documents. This is wierd, i had these exact flags yesterday and it wasn't working...